(1) Field of the Invention
The present invention relates to a one-component dry magnetic developer suitable for obtaining a copied image or printed image by developing a positively charged image on a photosensitive plate and transferring the developed image on a plain paper as a transfer sheet.
(2) Description of the Prior Art
A so-called one-component magnetic developer comprising a finely divided magnetic material dispersed in developer particles is known and widely used as the developer capable of developing an electrostatic latent image without using a particular carrier.
As one type of the one-component magnetic developer, there is known a so-called conductive magnetic developer in which a finely divided magnetic material is incorporated in developer particles to impart a magnetically attractable property and a conducting agent such as conductive carbon black on the surfaces of the particles to impart an electric conductivity (see, for example, the specifications of U.S. Pat. Nos. 3,639,245 and 3,965,022). When this conductive magnetic developer is brought into contact in the form of a so-called magnetic brush with an electrostatic latent image-supporting plate to effect development of the latent image, there can be obtained an excellent visbile image free of so-called edge effects or fogging. However, it is known that a serious problem arises when this developer image is transferred from the plate to an ordinary transfer sheet. More specifically, as disclosed in Japanese Patent Application Laid-Open Specification No. 117435/75, when the inherent electric resistance of a transfer sheet is lower than 10.sup.3 .OMEGA.-cm as in case of a plain paper, at the transfer step, broadening of the contour or reduction of the transfer efficiency is caused by scattering of developer particles. This disadvantage is removed to some extent if the toner-receiving surface of a transfer sheet is coated with a resin, wax or oil having a high electric resistance, but this improving effect is reduced under a high humidity condition. Furthermore, the cost of transfer sheets is increased by coating with a resin or the like and the touch or feel is reduced by the presence of such coating.
As another type of the one-component magnetic developer, there is known a one-component non-conductive magnetic developer comprising an intimate and homogeneous mixture of a finely divided magnetic material and an electricity-detecting binder. For example, the specification of U.S. Pat. No. 3,645,770 discloses an electrostatic photographic reproduction process in which a magnetic brush (layer) of the above-mentioned one-component non-conductive magnetic developer is charged by corona discharge with a polarity opposite to the polarity of an electrostatic latent image to be developed, the charged developer is brought into contact with an electrostatic latent image-supporting plate to develop the latent image and the developer image is transferred onto a transfer sheet. This electrostatic photographic reproduction process is advantageous in that a transfer image can be formed even on a plain paper as a transfer sheet. However, it is difficult to uniformly charge the magnetic brush of the non-conductive magnetic developer entirely even to the deep root portion, and therefore, it is difficult to obtain an image having a sufficiently high concentration. Furthermore, since a corona discharge mechanism should be disposed in the zone of a developing device, the copying apparatus becomes complicated.
Recently, there have been proposed a process in which development of an electrostatic latent image is accomplished by utilizing charging of a non-conductive magnetic developer by frictional contact of the developer with the surface of an electrostatic latent image-supporting plate (see Japanese Patent Application Laid-Open Specification No. 62638/75) and a process in which development is accomplished by utilizing dielectric polarization of a non-conductive magnetic developer (Japanese Patent Application Laid-Open Specification No. 133026/76).
In the former process, however, developing conditions should be controlled strictly, and if the developing conditions are not strictly controlled, fogging in a non-image area (especially conspicuous when the degree of contact of the top end of a spike of magnetic toner particles with the surface of a photosensitive material is high) or fixation or blocking of magnetic toner particles on a developing sleeve is caused, and this defect is especially conspicuous when the copying operation is continuously conducted.
In the latter process, although a problem of fogging does not arise, since a visible image is formed by the developing charge produced by the dielectric polarizing effect induced in the magnetic toner to an electrostatic latent image, a low-potential portion of the latent image cannot effectively be developed. Accordingly, a low-density area of an original is not effectively reproduced in the obtained copy and reproduction of a half tone in the copy is very difficult.
Furthermore, copies obtained according to these two known processes are poor in the image sharpness, and when a p-type photosensitive material such as selenium is used for a photosensitive plate and a positively charged image is developed, it is difficult to obtain images having a sufficiently high density according to any of these two known processes.